Regular paper. Determination of axial flux motor electric parameters by the analyticfinite elements method

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1 A. Moalla S. Tounsi Regular paper R. Neji Determination of axial flux motor electric parameters by the analyticfinite elements method This paper describes the electric parameters determination for an axial flux permanent magnets synchronous motor, using the joined method analytic/finite elements. Indeed several models of mutual and principal inductances and electric motor constant are developed analytically and validated by the finite elements method. These models are fortunately parameterised allowing to the formulation of several optimisation problems such us the motor ripple torque. KEYWORDS: Finite elements, analytic method, Mutual inductance, electric motor constant, torque, electromotive force. 1. INTRODUCTION Brushless permanent magnet motor operation consists in the conversion of energy from electrical to magnetic to mechanical. Because magnetic energy plays a central role in the production of torque, it is necessary to formulate methods for computing it. In deed there are numerous ways to determine the magnetic field distribution within an apparatus. For very simple geometries, the magnetic field distribution can be found analytically. However, in most cases, the field distribution can only be approximated. Magnetic field approximations appear in two general forms. In the first, the direction of the magnetic field is assumed to be known everywhere within the apparatus. This leads to magnetic circuit analysis, which is analogous to electric circuit analysis. In the other form, the apparatus is discretized geometrically, and the magnetic field is numerically computed at discrete points in the apparatus. This approach is commonly called finite element analysis. Permanent magnets Synchronous motors with axial flux (PMSMAF) were the subject of several research tasks which increasing in the last few years [1]. This increase is due to the advantages which the PMSMAF presents in structure, reduced volume, and as performance and the significant torque at low speed. The determination of the parameters of the PMSMAF is a precondition necessary for any applications of the electric motion [7]. In this paper, a model of the electric motor parameters is established in order to fix on the one hand variables influencing certain performances such as : ripple torque and motor converter consumption and on the other hand to use it in the variable speed control approaches.. MOTOR STRUCTURE The axial flux permanent magnets motor employed in this study is made with four poles pairs and six main teeth, between which six teeth are inserted in order to improve the electromotive wave-form and to reduce leakage flux []. Figure 1 illustrates a frontal view of the studied configuration. Laboratoire d Electronique et des Technologies de l Information LETI- Equipe Véhicule Electrique et Electronique de Puissance VEEP- Ecole Nationale d Ingénieurs de Sfax B.P. W, 3038 Sfax Tunisia

2 Slot North Permanent Magnet Main tooth Inserted tooth South permanent Magnet STATOR ROTOR Figure 1: Front view of the four poles pairs and six main teeth of the motor Each winding is composed by two diametrically opposite coils surrounding the main teeth. There are two configurations witch depends from back electromotive force (back E.M.F.) wave form witch can be sinusoidal or trapezoidal. 3. MOTOR MODELING Motor dimensions and electric parameters are calculated by analytical method [3]. To validate this dimensioning method, the motor is drawn according to its geometrical magnitudes extracted from the analytical model with the software MAXWELL-D [10] Analytical model of the motor We treated in this paper the analytical model of the motor with the trapezoidal wave form and the sinusoidal wave form Trapezoidal wave form structure The magnetic induction is supposed with a rectangular wave form into the air gap (figure ). Slot B e Main Tooth Slot North PM South PM Figure : Air gap induction in trapezoidal configuration

3 The electromotive force per phase value is proportional to the angular speed, the number of spire per phase and the flux variation, is given by the following expression: dφ dθ E = N. (1) dθ dt Dext Dint E N..Be.Ω 4 = () dω θ = (3) dt where: B e : induction in the air-gap, N : number of spires per phase, D int : internal diameter, D ext : external diameter, Φ : flux value, θ : angular speed. The electric constant is given by: N K e =.( Dext Dint ). B trap e (4) For trapezoidal supply currents in phase with the back E.M.F, the instantaneous power is equal to its average value [7]: P trap e =.E.I (5) Where E and I are respectively the maximum value of the back electromotive force and the maximum current value. The torque is given by: So, C Cm trap = K e.i ( 6) m trap D D ext int = N.( ). Be.I (7) Sinusoidal wave form structure The magnetic induction is showed in the figure 3. 3

4 Figure 3: Air gap induction in sinusoidal configuration When the motor is supplied with sinusoidal currents in phase with the back electromotive forces, the torque takes the following form: 3 Cem = K I sin e sin (8) where K esin is the electric constant of the back E.M.F. The maximum value of back E.M.F. at maximum speed is given by the following expression: E 3 = Ω max (9) phi K e The electric constant is given by: K 3 Dext Dint =.N.( Be (10) e ). sin Analytical model of inductance In this type of motor, the value of the inductance is low because the flux created by the coil crosses the air gap and the magnet thickness. For the inductance calculation, the motor is supplied by its peak current, and the magnets are replaced by air [8], then we obtain the flux lines distribution witch illustrated by the figure 4. Figure 4: Flux lines distribution around one stator pole 4

5 For a linear system, the inductance value of the phase constituted by two coils may be obtained from [5]: The energy calculation L =.A. Bm H.ds (11) I area The flux calculation N 1 L =..A. A. ds I A slot (1) Where Bm is the flux density, H is the magnetic field, Aslot is the slot area, and potential. The phase of the all configurations is expressed as follows: A is a scalar 3 0 Senc L = μ. N te e + t m A.H +.L d enc.n (13) Where, S is the slot area, H is the slot height, L is the slot width, e is the air gap slot thickness and N is the teeth number. te d enc 3.3. Analytical model of resistance The phase resistance is given by the following formulations respectively for trapezoidal and sinusoidal configurations [6]: R ph _ trap = ρ (Tb ).N.l sp.. δ (14) I c R ph _ sin = ρ (Tb ).N.l sp _ sin.. δ (15) I eff Where lsp _ trap and l sp _ sin are respectively the average whorl lengths for the trapezoid and sinusoidal configurations, I c is the trapezoidal peak current value and I eff is the sinusoidal effective current value. ρ is the copper resistivity at the winding temperature T b, it is expressed by the following function: ρ ( T ) = rcu[ 1 + α( T 0) (16) b b ] 4. SIMULATIONS The knowledge of electromagnetic fields when designing electric motor is primordial, considering the physical complexity of the phenomena which proceed there. This knowledge contributes to determine the losses and the forces for the realization of the electro-technical devices [6]. In this context, it is significant to have a model which represents correctly the reality and to calculate necessary flux used for the calculation of the 5

6 electric parameters of the motors. With this intention, the only methods available are those which use the finished differences or the finite elements. In this paper, we used the finite element method used by several computation fields softwares such as FLOW D developed in the E.N.S.I.E.G. of Grenoble, OPERA distributed by Vector Field and Quick Field de Tera Analysis and MAXWELL -D distributed by Ansoft Corporation in the United States and which is used in our study [7]. MAXWELL D software [5], is a dedicated for the motors electromagnetic fields analysis having complex structure. It uses the finite element method to solve the electromagnetic problems all while being based on a two-dimensional geometrical representation [4]. The software architecture can be schematized as the following figure 5. Geometrical D Parameter definition Specification of field calculation - Limits conditions - Magnetic elements characteristics Mesh maker Solver Post treatment & results exploitation Figure 5: Software architecture of calculation by finite elements 4.1.Motor parameters determination The electric motor parameters are carried out by the magnetic flux calculation developed by the motor. On the basis of a two-dimensional geometrical representation of cylindrical cross section on the average diameter of the motor (figure 6), stator windings are supplied with current in order to calculate magnetic flux values. Cylindrical cut plan transformation The engine can be studied in D by decomposition in cylindrical plans of cuts in order to reduce the time of simulation. The variation of surface between magnet and a principal tooth is not linear because of the principal edges. So that, in order to find the true variation of flux, it is necessary to study the motor on several plans of cylindrical cuts. In order to use the analytic method and design tool adapted for preliminary design of permanent magnet axial flux machines, we begin by a two-dimensional study on a cylindrical section of the motor boring or average diameter illustrated by the figure 6. 6

7 Figure 6: Two-dimensional representation in cylindrical cut Axial/radial transformation The study on the axial flux motor can be replaced by its equivalent with radial flux. In facts, the transformation is carried out on an average contour of the engine with axial flux. To have the same back E.m.f., it is necessary that the average diameter of the axial structure is equal to the average diameter of the motor with radial flux Dext + Dint D = ( ), and the length of the radial motor Lm is equal to Dext Dint [9]. 4.. Simulation results The flux at load and at no load calculated by finite elements is illustrated by figures 7.a and 7.b for trapezoidal and sinusoidal configurations respectively. Flux at no load Flux at load Figure 7.a: Three phase flux (Trapezoidal configuration) Flux reaches the maximum value calculated by the analytical method which validates this approach. Flux is perfectly linear, which leads to obtaining perfectly trapezoidal back E.m.f. (figure 8.a.) for the trapezoidal configuration and sinusoidal back E.m.f. (figure 8.b.) for the sinusoidal configuration. This motor property reduces considerably the torque undulations. 7

8 Flux at no load Flux at load Figure 7.b: Three phase flux (sinusoidal configuration) Back E.m.f. at no load Back E.m.f. at load Figure 8.a: Three phase Emf (Trapezoidal configuration) Back E.m.f. at no load Back E.m.f. at load Figure 8.b: Three phase back E.m.f. (Sinusoidal configuration) 8

9 The electromagnetic torque is calculated by this equation: T m 1 = Ω m () t ei () t ii () t i = 1 (17) where e i is the Emf value of the i ème phase. From the Back.E.M.F we can determine K e. Figures 9.a, 9.b, 10.a and 10.b illustrate respectively the field lines distribution at no load and at load for trapezoidal and sinusoidal configurations. (a) Trapeze at no load (b) Trapeze at load Figure 9: Field lines distribution for trapezoidal configuration (a) Sine at no load (b) Sine at load Figure10: Field lines distribution for Sine configuration The field lines distribution shows that there is no magnet leakage. This property is obtained by geometrical optimization obtained by finite elements simulations [4]. The winding flux calculation of the first and the second phase by the finite elements method allows us the determination of phase inductance L and the mutual inductance M. ph By using the formulations (18) and (19) we obtain the values of L and M : ph L ph N. Φ p = (18) I 9

10 . M M N Φ = (19) I where: I : current nominal one, : phase inductance of the motor, L ph M : mutual inductance by phase of the motor, Φ p : magnetic flux of the fed whorl, Φ : magnetic flux of the next not fed winding, : conductor number per whorl by M phase, The two figures 11.a and 11.b shows that the rotation of the rotor by an angle θ, has no influence on the inductances values for the trapezoidal and sinusoidal configurations. This result validates the analytical model. N (a) Trapezoidal configuration (b) Sinusoidal configuration Figure 11: Invariance of inductances Table I illustrate inductance values obtained by finite elements method (FEM): ph MEF TABLE I: Values of inductances and the mutual inductances Trapezoidal configuration Sinusoidal configuration L (mh) L (mh) ph analytic M (mh) ph MEF M (mh) ph analytic 10

11 These results enable us to validate the values of analytically calculated inductances. The backs E.M.F at no load and at load values enable us to determine the torque variations according to the swing angle. The figures 1.a and 1.b, illustrate the torque variation of motor according to the swing angle at load and at no load for trapezoidal and sinusoidal configurations. (a) Trapezoidal configuration (b) Sinusoidal configuration Figure 1: Torque variation according to the swing angle 5. CONCLUSION In this paper we presented a calculation method of the electric parameters for trapezoidal and sinusoidal configuration of a permanent magnets synchronous flux with axial flux by finite element method. The results obtained for the trapezoidal and sinusoidal configuration validate the analytical model. Indeed in the first time we have determined and validated the analytical values of flux, electromotive force and torque calculated by the finite elements simulations, in the second time we have determined and validated the analytical values of inductance and mutual inductance calculated by the finite elements simulations. This work developed will allow us to make possible control and optimization studies for these types of motors. References [1] A. PARVIAINEN, J.P YRHÖNEN, M. NIEMELÄ, Axial flux permanent magnet synchronous motor with sinusoidally shaped magnet, ISEF 001. [] A.CAVAGNINO, F.PROFUMO, A.TENCONI, Axial flux motor: structures and applications, Electro motion 001 pp. 5-14, June 19-0, 001, Bologna. [3] S. BRISSET, P. BROCHET, Shape Optimization of BDC Wheel Motor using Powell s Method, COMPEL vol. 19, No., pp , July

12 [4] N.CHAKER, S.TOUNSI, R.NEJI, F. SELLAMI, Design and modeling by the combined method analytic/finite elements of permanents magnets axial flux motor, International Congress on the Engineering of Renewable Energies. November 6-8, 006, Hammamet - Tunisia, WEN04. [5] S. TOUNSI, R. NEJI, F. SELLAMI, Design of a radial flux permanent magnet motor for electric vehicle, 10th European Conference on Power Electronics and Applications, September 003, Toulouse-France, CD: P151. [6] M.BOUHARKAT, Etude de l évolution des courants rotorique d une motor asynchrone à cage en régime dynamique, Thèse de Doctorat 006, Université de Batna Algérie. [7] S. TOUNSI, Dimensionnement et modélisation d un moteur synchrone à flux axial pour la propulsion de véhicule électrique de loisirs, Mémoire de DEA électronique 001, Ecole Nationale d Ingénieurs de SFAX - TUNISIE. [8] G. MADESCU, I.BOLDEA, T.J.E. MILLER, The optimal lamination approach to induction machine design global optimisation, IEEE Transaction on industry applications, Vol. 34, No. 3, pp , May/June [9] G. MADESCU, I. BOLDEA, T.J.E. MILLER: The optimal lamination approach to induction machine design global optimisation: IEEE Transaction on industry applications, Vol.34, No. 3, pp , May/June [10] MAXWELL, Ansoft Corporation, four station square suite 660 Pittsburgh PA 1519, USA. 1